Polyethylene terephthalates have been used in a variety of applications since it has excellent mechanical properties, thermal properties, chemical resistance, electrical properties and moldability.
However, since the mechanical and physical properties of polyethylene terephthalates are impaired by hydrolysis, a variety of studies have been conducted to inhibit such hydrolysis in cases where a polyethylene terephthalate is used over a long period of time or in a humid condition. Particularly, in solar cell films, since their outdoor service life is required to be at least 20 years, high hydrolysis resistance and elongation retention rate are required.
For example, JP 2008-31439 A discloses a technology for reducing the amount of COOH terminal groups and improving hydrolysis resistance by compounding a cross-linked component having at least 3 functional groups and a sequestering agent using a twin-screw extruder.
Further, JP 2007-277548 A discloses a method of producing a polyester containing inorganic phosphate and in the examples thereof, phosphoric acid is used in combination.
JP 2008-7750 A discloses a polyethylene terephthalate containing a buffer phosphorus compound and in the examples thereof, the polyethylene terephthalate is used in combination with a phosphorus compound.
However, although the technology of JP '439 is excellent in improving the initial hydrolysis resistance, since the technology uses no phosphorus compound, hydrolysis reaction proceeds acceleratingly, so that it is difficult to satisfy long-term hydrolysis resistance. In addition, since a cross-linked component is compounded, reaction occurs ununiformly and minute gel-form foreign substances are likely to be generated, so that the technology of JP '439 is not suitable for film applications.
Further, in the technology of JP '548, since the ratio of phosphoric acid and inorganic phosphate and the applied amounts thereof are inappropriate, inorganic phosphate is likely to be converted into a foreign substance and, although the short-term hydrolysis resistance is excellent, long-term hydrolysis resistance, which is required for use in solar cells and the like, is not sufficient, so that the mechanical and physical properties of the film are deteriorated by such foreign substance.
In the technology of JP '750, since the type, ratio, applied amount and the like of the phosphorus compound are not sufficiently and properly adjusted, the resulting hydrolysis resistance and mechanical properties are not sufficient for solar cell applications.
It could therefore be helpful to provide: a polyethylene terephthalate composition overcoming the above-described drawbacks, which has excellent hydrolysis resistance and retention of elongation and is suitable for use in a solar cell film; and a polyethylene terephthalate film composed of this composition.